1. Field of the Invention
This invention relates to improvements in air brake control systems for controlling the braking function of multi-axle heavy duty vehicles, including tandemly towed vehicles, and, more particularly, relates to an improved parking/emergency spring brake control valve for use with a trailer, or trailers, being towed behind a tractor. The emergency control valve of the present invention prevents release of the spring brakes during initial charging of the braking system's service reservoir until such time as the pressure within the reservoir has reached a predetermined minimum and wherein upon a failure of the supply pressure within the braking system, the pressure within the spring brakes will be permitted to exhaust to atmosphere thereby causing immediate application of the spring brakes. In one embodiment of the present invention, the emergency control valve assembly permits pressure to be applied or bled directly to the spring brake chambers to thereby release the spring brakes and allow emergency towing of a trailer when it is not otherwise possible to pressurize the trailer's service reservoir.
2. History of the Related Art
The present emergency control valve is designed to provide control of a trailer's spring brakes as required by DOT (Department of Transportation) FMVSS No. 121 as modified by Docket No. 90-3, Notice No. 2. This Notice eliminated the requirement for a "protected tank" for release of the spring brakes of a tractor trailer air brake system in emergency situations. The notice also required that no single leakage type failure in the service system, such as a rupture of the service reservoir, would result in supply line pressure falling below a threshold pressure of 70 psi. The regulations further required that spring brakes not provide any drag when the supply line pressure is 70 psi or greater. One type of spring brake control valve system is disclosed in U.S. patent application Ser. No. 07/830,672 filed Feb. 4, 1992 in the name of Edward Sell and entitled "Air Brake Control Valve System".
In conventional tractor trailer air brake systems, pressurized air is supplied from the tractor to the trailer braking system by way of a service brake (control) line and an emergency brake (supply) line. The control line is connected through a relay valve to pneumatically operate service brakes associated with each wheel of the trailer. In order to assure that the service system has adequate volume to provide for repeated service brake applications, the braking system further includes a service reservoir or tank which is connected through the relay valve so that pressure may be supplied to the service brakes from a relatively large storage volume. The reservoir is pressurized through the supply (emergency) pressurization system which also provides pressure to the emergency or parking brakes of the vehicle. The emergency or parking brakes are generally spring operated with the force of the springs being offset to release the brakes by the use of pressure within the emergency supply system.
In the aforementioned U.S. application Ser. No. 07/830,672, now U.S. Pat. No. 5,172,958, control of supply air to the emergency/parking brakes is provided by special control valve situated between the parking brakes and the service reservoir. The control valve functions to regulate service and supply air to insure maximum air pressure being delivered to the spring brakes and to allow filling of the system's reservoir during normal operation of the air brake system. The control valve further regulates the release of air pressure in the spring brake system when pressure in the supply system drops below a predetermined minimum isolation pressure and prevents compounding of forces in the brake system by venting service air to atmosphere when the spring brakes are applied. The system was designed to meet each of the requirements set forth by the Department of Transportation. As a result of meeting such requirements, under circumstances when the service reservoir of a trailer is being initially pressurized, such as after the reservoir has been intentionally de-pressurized to remove contaminants or when it has become de-pressurized due to leakage when the system is not in use, the parking brakes are released momentarily (for a few seconds) prior to the service reservoir being pressurized to a minimum threshold level. During this period of time, the service brake system of the towing tractor provides braking for the tractor and trailer. Some tractor trailer operators believe that it is important that the spring brakes must not be released, at all, until the pressure within the service reservoir reaches an operable level, even if this would require the mechanical release of spring brakes by caging techniques.
If the air pressure which has been employed to release the emergency brakes is lost, such as when an accident involves the disruption of the air supply to the trailer, the brakes are automatically applied by spring pressure. Common causes of such failures are damaged air connectors or gladhands, damage to the air lines from the connectors to the supply reservoir, damage to the reservoir, failure of a brake chamber, and the like. Under each of these conditions, the brakes are automatically set by spring pressure preventing the vehicle from being moved without mechanically caging the springs by use of a special tool.
To alleviate the problem of mechanically caging the brakes to allow emergency movement of a vehicle under circumstances where the service reservoir is unable to retain pressure, the aforementioned U.S. application Ser. No. 07/830,672 now U.S. Pat. No. 5,172,958 discloses a control valve which enables supply air to be utilized to disengage the emergency or spring brakes allowing a disabled vehicle to be quickly moved from a dangerous location. The valve functions to prevent air supply from flowing into the system's reservoir at any pressure lower than the pressure required to release the spring brakes. Therefore, whenever air is supplied through the control valve, the air is simultaneously made available to the spring brake chambers. There are those however, who do not desire a direct supply of air to the spring brakes prior to providing sufficient reservoir pressure. However, under circumstances where the reservoir has itself been punctured or damaged, emergency towing has not been provided for utilizing conventional systems.